Conventionally, a radio broadcast receiver has been known which receives broadcasting stations with different frequencies in the same receiving band using main and sub two tuners, respectively.
As for a broadcasting signal a radio broadcast receiver receives, tuning is done by adjusting the oscillation frequency (local oscillation frequency fL O C) of a local oscillator in a front end.
How much the local oscillator lowers the frequency f of the received signal depends on whether the frequency is higher or lower than the local oscillation frequency fL O C. In either case, the intermediate frequency fI F becomes fI F=|f−fL O C|. Accordingly, the frequency that is likely to become the local oscillation frequency fL O C is two: fL O C=f+fI F and fL O C=f−fI F. The former is referred to as an upper local oscillation frequency and the latter is referred to as a lower local oscillation frequency.
However, when the intermediate frequency fI F is smaller than the receiving bandwidth (upper end frequency−lower end frequency), the following problem occurs when trying to receive different broadcasting stations within the same receiving band by the two tuners. More specifically, regardless of whether the upper local oscillation frequency or the lower local oscillation frequency, when the local oscillation frequency of a first tuner agrees with the receiving frequency of the second tuner or if it crosses the receiving frequency of the second tuner when altering its tuning, noise is mixed in the second tuner owing to interference and receiving performance is lowered.
For example, as shown in FIG. 7, on the assumption that the receiving bandwidth is 76 MHz-90 MHz, the intermediate frequency fI F is 10.7 MHz and the receiving frequency of the first tuner is 89.4 MHz, although the upper local oscillation frequency (upper local oscillation) which is 89.4+10.7=100.1 MHz is beyond the band, the lower local oscillation frequency (lower local oscillation) which is 89.4−10.7=78.7 MHz falls within the band. Thus, when trying to receive a broadcasting station within the same receiving band by the second tuner, the interference due to the local oscillation frequency can occur.
In view of this, to prevent the local oscillation frequency of the first tuner from interfering with the receiving frequency of the second tuner, Patent Document 1 discloses a conventional technique that stops the second tuner from receiving at the frequency when the local oscillation signal of a particular receiving frequency of the first tuner and its harmonics agree with the receiving frequency of the second tuner.
Prior Art Document
Patent Document
Patent Document 1: Japanese Patent Laid-Open No. 1-132968 (1989).
According to the technique disclosed in the foregoing Patent Document 1, since the second tuner cannot continue receiving, a problem arises in that the double tuners consisting of the main and sub two tuners cannot achieve the original performance.
The present invention is implemented to solve the foregoing problem. Therefore it is an object of the present invention to provide a radio broadcast receiver capable of enabling the main and sub two tuners to continue their reception by preventing the local oscillation frequency from interfering with the second tuner beforehand.